Investigations On Fire Smoke Spread And Buoyant Flow Stratification In A Longitudinal Ventilated Tunnel With Ceiling Extraction

A series of experiments are conducted in a model tunnel to study the thermal smoke back-layering flow length and the smoke thermal stratification with ceiling extraction in a longitudinal ventilated tunnel and the front velocity of ceiling jet with different ceiling extraction velocities.The specific work are as follows:This paper investigates thermal smoke back-layering flow length beneath the ceiling in a tunnel with combination ceiling extraction and longitudinal ventilation in case of the ceiling extraction at the upstream of the fire source.A series of experiments are conducted in a scale model tunnel.The thermal smoke back-layering flow length will be smaller with the ceiling extraction at the upstream of the fire source.A factor is put forward to representate the influence of ceiling extraction.A new non-dimensional factor is proposed to predict the thermal smoke back-layering flow length,which is shown to be in good agreement with the experimental results.In this work,the ceiling jet front velocity in a tunnel with ceiling mechanical smoke extraction system is studied by a set of model-scale tunnel experiment with ceiling extraction system.And some classic model predictions of ceiling jet front velocity are compared with experiment measurements under the effect of different ceiling extraction velocities.Meanwhile,the relative location between fire source and ceiling extraction are considered.The Hinkley's model which consider heat loss from smoke flow can be well used to predict the front velocity of ceiling jet with different ceiling extraction.A series of experiments are carried out in a model tunnel to investigate the buoyant flow stratification behaviors in a tunnel fire with combination of longitudinal ventilation and ceiling extraction,which has not been studied before.The interaction between them can be expressed by the Froude number or Richardson number.At regime I(Ri > 2.0 or Fr < 0.66),the buoyant stratification is stable and with a clear interface between the upper buoyant flow layer and the lower air layer.At regime II(1.4 < Ri < 2.0 or 0.66 < Fr < 0.8),the buoyant stratification is basically stable,but some vortexes exists at the interface.At regime III(Ri < 1.4 or Fr > 0.8),a strong mixing exists between the upper buoyant flow and the lower air flow,and the buoyant flow stratification becomes unstable.